Standard SFS 5511 AIR CONDITIONING. INDOOR CLIMATE IN BUILDINGS. FIELD MEASUREMENTS OF THERMAL PARAMETERS. 9. TECHNICAL MEASUREMENTS OF THE SYSTEM. GENERAL INFORMATION.
The capability to execute temperature parameters of air conditioning system according to targets
and plans, must be discovered by measurements of the following magnitudes:
-
air temperature
-
air humidity
-
air flow velocity
-
system pressure conditions and pressure differences
-
devices’ wattage
9.2. Measurement planning
Measurements done in air conditioning system must be taken into account already at systems’
design phase.
Often big impermanent and local changes of different conditions occur in air conditioning systems
and air conditioning processes. Also many other factors adding to the uncertainty of the
measurement, can occur.
System measurements’ and air flow velocity measurements’ accuracies can be improved by
improving the flows’ smoothness. In measurement planning the following must be taken into
account:
-
stratified temperature (and usually uneven temperature distribution) is at it’s biggest after
heaters, coolers, heat recovery units and mixing sections. Usually in that case temperature
measurement requires also protection against radiation.
-
temperature measurement of air where drops or fog humidity occur, must be avoided. -
parameters’ (temperature, humidity, air flow velocity) impermanent changes require
recording of measurements, for example by multipoint plotter. In measurement such devices
must be used, which time constant is small compared to velocity of the changes.
-
magnitudes (temperature, air flow velocity, sometimes humidity or/ and pressure), which
have influence on components and devices’ performance, must be measured in the same
time.
Fixed or movable measuring devices are used for condition measurement in air conditioning
system. If building is in use, reliability of fixed measuring devices must be checked, for example,
by comparative measuring by calibrated measuring instruments.
10. AIR TEMPERATURE TECHNICAL MEASUREMENTS OF THE SYSTEM.
10.1. Measuring devices
Measuring sensors of the same type as for a room temperature measurement are also suitable for air
temperature technical measurements of the system, unit 4.2. When measuring the systems’ and the devices’ characteristic curve values, the scale-division value
should be maximum 0,2°C. Measuring instruments often used for operational checking of the
system (most often fixed ones), scale-division value should be maximum 1°C. And in this case,
they cannot be used for exact measuring.
10.2. Measuring performance
If air temperature is even-tempered, as it usually happens after fans, one measuring point is enough.
In stratified air, in which flow velocity varies, temperature and flow velocity must be measured at
the same time. Average temperature Tm can be gotten by measuring temperatures and flow
velocities in different measuring points:
Tm 
where
T1  v 1  T2  v 2  ...  T  n v n
v 1  v 2  ...  v n
(1)
T1, T2, … Tn are local temperatures, which are measured in different points
v1, v2, … vn are flow velocities, which are measured in the same points (1…n)
The number of measuring points can be defined according to picture 10. Measuring can be started,
for example by 5 points-method according to standard 5512. If the difference between the highest
and the lowest temperature is less than 10% of the biggest temperature difference (measurements
must be done in different parts of measured device), there is no use for more points. If temperature
difference is 10---20%, 4 points more must be added. If temperature difference is more than 20%,
5-10 points must be added. Extra points are chosen according to the velocity distribution.
Measuring can also be done by using the sampling tube, according to standard SFS 5404.
Measuring can be repeated for double-checking.
Radiation protection must be done according to picture 3. For running air the protection pipe of
small diameter is enough. Radiation protection should not create air flow disturbance.
Picture 3
11. HUMIDITY MEASUREMENTS IN AIR CONDITIONING SYSTEMS
Measurements are planned and carried out according to unit 10. For humidity measurements, most
often, wet bulb temperature measurement by psychrometer or by different dry and wet bulb
temperature measuring sensors are used. Demands for measuring performance and devices’
restrictions must be represented in instructions. In air conditioning system humidity must be
measured in a reliable way, for example after fan.
Air flow humidity measurements are done for example as wet bulb temperature measurement
without booster fan or by sampling tube. Dry and wet bulb temperature must be measured in the
same place.
12. AIR FLOW VELOCITY IN AIR CONDITIONING SYSTEMS
12.1. General information
Air flow velocity measurement must be done for air flows’ defining, for checking the temperature
distribution results and for measuring of components wattage. Sometimes measuring has to be done
in uneven flow field (unit 12.2.)
Picture 9. Example of choosing the measuring points, stratified flow.
The exact determination of measuring points must be done according to the methods, which are
presented in picture 10.
5 points-method
Log-linear-method
Rectangle-method
Log-Tschebyschew-method
Picture 10
12.2. Measuring devices.
The following are suitable for air flow velocities measurement:
-
pitot-pipe and pressure differences measuring devices, when reasonable measuring accuracy
requires at least 3 m/s air flow velocity. Measurement can be done by multipoint-method
according to the standard 5512, in picture 10.
-
hot-wire anemometer or, in special situations, blade wheel anemometer. Measuring
instrument must be constructed according to velocity range; so called direction-independent
sensors are not usually suitable for duct measuring.
13. PRESSURE DIFFERENCE MEASUREMENT
Pressure difference measurement is described in standard 5512. 14. MEASUREMENT OF COMPONENTS’ WATTAGE
14.1. General information.
From the point of view of temperature conditions the following measurements are needed:
-
measurement of fan power and efficiency
-
measurement of components’ heating and cooling
-
measurement of heat recovery unit’s efficiency
-
measurement of humidification ratio
Field measurements can be conducted extremely rarely for laboratory tests according to test
standards.
In the above mentioned tests instructions of this standard can be applied for measurement of
temperature, humidity and air flow velocity.
14.2. Power of heating or cooling
Air temperature and air flow velocity are measured before and after components, in units 9, 10 and
12 according to the instructions.
Usually temperature is distributed unevenly, so radiation protection is needed for temperature
measurements.
Power φ (W) can be calculated by the following equation:
φ  A  v m  (T1  T2 )  ρ  c p
(2)
where A is area of cross-section in measurement point, m2
vm is average value of measured velocities, m/s
T1 is average value of temperature before component, calculated by equation 1(unit 10.2.), K
T2 is average value of temperature after component, calculated by equation 1(unit 10.2.), K
ρ is density of air, kg/ m3
cp is specific heat capacity of air, J/kg K
14.3. Fan power and total efficiency
The following is to be measured:
-
power consumption of fan motor,
-
air flow velocity in order to estimate an air flow
-
total pressure difference of fan.
Measurement must be done according to the standard 5147, unit 7.2.4.
Generally, power measurements have to be based on the assumption that the factory’s performance
characteristics and the same, real motor’s performance characteristics will correspond to each other
exactly enough.
In all cases, electric current consumed by a real motor must be the same as in factory’s performance
characteristics. The voltage can vary maximum 2% from average value during measuring.
Accuracy class of voltage, electric current and power measurement are chosen according to
accuracy requirements of measurements.
Total fan efficiency, according to power consumption of fan motor, must be calculated under
standard SFS 5147 by the following equation:
ηt 
where
q V1  Pt F
qv1 is fan volume flow, m3/s
ptF is fan total pressure, kPa
PE
(3)
PE is power consumption of motor, kW
14.4. Heat recovery unit’s temperature efficiency
Temperature must be measured in different parts of heat recovery unit, picture 11. Usually
temperature is distributed unevenly, so measuring sensor must be protected from radiation. If air
flows are equal, temperature efficiency ηt is calculated by the following equation:
ηt 
t 2  t1
t 3  t1
(4)
where t1,t2 and t3 are air temperatures in different parts of heat recovery unit, picture 11.
If air flows are not equal, in addition to temperature efficiency, the proportion of outdoor air flow to
extract air flow must be mentioned.
If needed, also air flows, air humidity and pressure differences above the heat recovery unit and
between supply and exhaust sides must be measured. Device’s freezing susceptibility must be
estimated according to type approval instructions for heat recovery units. During measuring there
must be no air leakage.
1- Incoming outdoor air to heat recovery unit (cold air)
2- Out coming supply air (heated incoming air from HRU)
3- Incoming exhaust air (from building to HRU)
4- Out coming extract air (from HRU to outside)
Picture 11